Preparation of lithographic plates



PatentedjDec. ll,

" UNITED STATES PATENT osncsv.

raaransrron or: mooaarmcmrns cranes 1-. King, cinnamwomo, assignor'to The Lith raphic 'leohnioal Foundation, Ine., New York N. I.,'aeorporation of Delaware v v No unwind Application February 11,1944, j sermmszaozs 7 Claims. (on on-react m. invention relates humans and a directed particularly to improvements in the art of making positive reversal and deep-etch litho graphic plates.

The conventional process of making a deepetched lithographic plate consistsin coating a.

grained metal plate with a mm or dichromated colloid, exposing this illm to a light image 01 the subject to be reproduced, developing to remove the unherdenedillm from the image areas, etchin! the metal suriace slightly in the image areas,

applying an. ink or ink-receptive film to the etched image areas, then removing the light hardened colloid film or stencil from the nonimage areas, and finally treating the non-image areas to render them receptive to moisture and non-receptiveto ink.. g

In makings positive reversal lithographic plate these steps are exactly the same except that etching of the plate following the development is when the light sensitive coating consists of dichromated glue development is accomplished by washed with an anhydrous alcohol and dried and.

next, ink-receptive film is applied to the etched Carcass-,-

Lithographic plates oieither of these types, while constituting an appreciable improvement over plates made by other methods, requireconsiderable care in preparation, and occasionally they fail for lack of abiilty to retain a satisfactory ink-receptive image. The reasons for such falls ure are usually brought about through incom-- plete removal of hygroscopic salts from the etched image areas or incomplete drying of the etched image areas. Under these conditions the ink or ink-receptive illm is inadequately bonded to the metal surface and during printing becomes detached as a result of the action of the moisture. The plate is then said to have become *blind. Unfortunately this trouble cannot be result that s new memcamdtjaatotsiderabletimeislost.

f In the case of plates coatediwith dichro'mated gumarabic it has been recently to remove theresidue of, deep etching solution, and v at the same time remove the resist by washing the plate inhot water, the purpose being to eliminate 'the use of the anhydrous alcoholic wash,

while washing with water is no. more deleterious to plate performance than washing with anhydrous' alcoholf'still failures of the same type do occur because of the dimculty in removing the IaStinvisibIetr'aces' of moisture fromthe surfaces in 'thegimage areas. This dimculty is especiallyipronounced during periods of high at- 'mospherlc humidity. As a result neither washreceptive lacquer which ,is moisture absorbent whereby the plate is dehydrated or immunized against possible deleterious eilects' caused by traces of water' at the etched surfaces. More spes ciflcally, the invention contemplates treatment with a lacquer containing a substantial portion of an anhydrous solvent which is capable of formins. with water, a minimum boiling point miX- ture, whereby the lacquer is endowed with the j function, first, of exerting a drying eflect upon detected until the plate a on the press with the the surface to which it is deposited, and then protecting that surface upon evaporation of the solvent-water mixture. This dehydrating lac-' quer enables consistently satisfactory results to be-obtained under all weather conditionslwhether' the plates have been subjected to an alcoholic or cold waterwash for removal of the deep-etching solution, or to a hot water washfor removal .of

both the etching solution and resist. I A representative composition having thee; properties consists essentially of a suitable film forming resin and anhydrous isopropyl alcohol as the lacquer solvent; The isopropyl alcohol I forms an azeotropic mixture with water, releasing vapors which, condensed, may contain as much as 9% water; otherwise expressed, the vapors given of! are richerin water than the unvapormoisture remains to interfere with the bond between the resin film and the metal surface. It

by absorption and the evaporation which occurs does not cool the plate sumciently to cause it subsequently to condense moisture. Moreover, the residue after evaporation is non-hygroscopic.

The dehydrating lacquers of the, present -invention contain a substantial portion of an an drous solvent in which the film forming resin is soluble and which itself is capable of forming an azeotropic solution with water, but it also has been determined that the solvent component of the lacquer system may contain appreciable percentages of various aromatic hydrocarbons, which sometimes are employed in lacquer formulation, without adversely affecting the dehydrating characteristics of the composition. In such instances ternary azeotropic mixtures are formed; in any event, a favorable rate of evaporation is displayed. the water present being evaporated along with the volatile organic solvents. Therefore, upon final drying of the resin film, the plate is freed from water and an ink-receptive base is provided even though the plate was moist when the lacquer was applied.

There are many resins which may be used as the film forming components of the lacquer systems including cellulose ethers, resins of the alkyd type, chlorinated rubbers and the like. While it is impossible to describe in detail all of the ,various film forming resin and solvent systems which may be used, three primary factors govern their selection. First, the resin should leave a film which is receptive to ink after the solvent has evaporated from it; second, the resin should be truly or colloidally soluble in. anhydrous solvents capable of forming constant boiling mixtures with water; and any water present upon the plate when the lacquer is applied should leave the film by evaporation along with the solvent. Representative examples of compositions disand as a practical matter evaporates somewhat too rapidly. An isopropyl alcohol azeotrope contains 9% of water and isopropyl alcohol therefore is more suitable as a dehydrant. Pyridine evaporates even more rapidly than ethyl alcohol and is also more expensive, and irritating to some people. On the whole, isopropyl alcoholor mixtures of isopropyl alcohol with other solvents are preferred. From the foregoing discussion of the principles of the invention, those skilled in the art readily will comprehend other materials which may be used to serve their function.

The lacquers of the present invention are employed in'the. usual way in the preparation of lithographic plates. The plates thus may be washed with anhydrous alcohol if the convenplaying'these properties are as follows:

. 1 Isopropyl alcohol 64 Teglac 128--alcohol soluble maleic resin"--- 16 Xylol i 20 2 I Isopropyl alcohol 45 Ethyl cell a 5 Solvent naphtha (coal tar naphtha) 50 3 Isopropyl alcohol 65 Petrex SS. alcoholsoluble alkyd 17 /2 Xylol 17 /2 4 Pyridine 40 Tornesitechlorinated rubber 1O Xylol 50 pyl, toluol plus isopropyl, and the like. An azeotrope of ethyl alcohol contains only 5% of water ftional practice is observed, to efiect removal of .washed'with hot water to cleanse them of'etch solution and at the same time remove the bichromated colloid film in which event the plates also are blotted to substantial dryness and then lacquered. If it is desired to preserve the'bichromated colloid film until after the application of the developing ink and'still not use anhydrous alcohol then a suitable procedure is to wash the plates in cold water to removc'the etching solution, dry the plates substantially, apply the lacquer and developing ink, and finally remove the resist.

Having described my invention, I claim:

1. The method of preparing a lithographic plate, which comprises the steps of removing deepetching'solution from.v the plate after it hasbeen etched, drying the plate until it is substantially free of water and then effecting final removal of water from the etched areas of the plate'by applying thereto a film forming lacquer containing a substantial portion of, an anhydrous organic solvent which forns a negative azeotropic mixture with water residing on the'plate. and effecting removal of the water from'the lacquer by evaporation of the azeotropic mixture.

2. A method of dehydrating the etched area of a lithographic plate which has residual water thereon and establishing an ink-receptive lacquer film upon the plate, which method comprises applying to said etched area a lacquer consisting essentially of an anhydrous organic solvent which is'capable of forming a negative azeotropic solution with water, and a film formingresin, forming .a negative azeotropio solution between said lacquer and said residual water the plate. and then dehydrating the plate and drying the lacquer by evaporating the azeotropic mixture of lacquer solvent and 'water absorbed from said plate.

. 3. A method of dehydrating the image areas of a lithographic platejwhich hasresidual water thereon, and establishing an ink-receptive lacquer film upon the plate, which method comprises applying to said etched areas a lacquer consisting essentially of a film forming resin and a combination of solvents capable offorming an azeotropic mixture with water having a minimum boiling point belowthat Many of said solvents and substantially. below that otwater, thereby 4. The method 'of preparing a lithographic plate which has residual waterthereon after etching, which method comprises drying the plate until it is substantially free of water, then eilecting final removal of water from the etched areas of the plate by applying thereto a film-forming lacquer containing an anhydrous lacquer solvent which forms a negative azeotropic mixture with water residing on the plate and which also contains a coal tar solvent, and eil'ecting removal of the water from the lacquer by evaporation or the azeotropic mixture.

5. The method of preparing a lithographic plate which has residual water thereon after etching, which method comprises drying the plate until it is substantially free of water, then eifecting final removal of water from the etched areas of the plate by applying thereto a film-forming lacquer containing a substantial portion of anhydrous isopropyl alcohol which forms a negative azeotropic mixture with water residing on the plate, and eilecting removal of the water from the lacquer by evaporation of the azeotropic mixture.

6. The method of preparing a lithographic plate which has residual water thereon after etching, which method comprises dryingthe plate until it is substantially free of water, then eflecting final removal of water from the etched areas of the plate by applying thereto a film-forming cellulose ether lacquercontaining a substantial portion of anhydrous isoprdpyl alcohol which forms a negative azeotropic mixture with water residing on the plate, and effecting removal of the lacquer from the water by evaporation of the azeotropic mixture.

7. The method of preparing a lithographic CHARLES F. KING. 

